1. Field of the Invention
The present invention relates to pressure and strain sensitive devices and, more particularly, to an improved strain gage pressure sensor.
2. Art Background
Proper operation of many systems requires a precise sensing and measurement of pressure. Pressure sensors have been developed which employ a single crystal material for the pressure sensing diaphragm. Typically, such pressure sensors employ the strain gage principle to transduce the sensed pressure to a useful electrical signal. In order to mount the sensor chip so that it may be packaged usefully, the chip is usually bonded to a support by means of epoxy, glass or electrostatic bonding or similar bonding methods. When pressure is applied to the diaphragm, strains are set up in the diaphragm surface and the strain sensitive regions are disposed appropriately to respond electrically to them. The pressure source may be any of a number of sources such as the pressure in an engine which impinges upon and exerts a pressure against the diaphragm of the sensor.
The strain gauges disposed on the diaphragm are usually arranged in the form of a Wheatstone Bridge circuit which is well known in the art, and which is coupled to an external voltage sensitive device. When the pressure impinges the diaphragm, moments are established in the body of the diaphragm which deflect it and relate directly to strain, resulting in a change of resistance of the strain sensitive portion of the diaphragm making up the Wheatstone bridge. This change is sensed and corresponds to the change in pressure which caused the deformation of the diaphragm.
However, serious problems exist in such prior art strain gauges in that radial and tangential stress caused by the pressure or strain being measured causes the glass, epoxy or other bond which couples the chip to the base to deform to the point where the diaphragm cannot return to its original shape, thereby causing the gage to go out of calibration. In response to such stress, the diaphragm may also separate from the base.
Over the course of operation of the strain gage, the bonding material may become permanently deformed, thereby requiring replacement of the gage which, since such gauges are extremely expensive, also incurs a considerable expense.
Since the purpose of such devices is to measure pressure or strain accurately and consistently over the course of years of usage with no down time, it will be appreciated that a dire need exists for a strain gage that overcomes the afore-discussed problems which are attendant prior art strain gauges.